This invention relates to an arrangement and a method for reducing distortion of an output signal provided by signal handling means in response to an input signal.
Current and forthcoming telecommunications standards place an increasingly stringent requirement on the linearity of transceiver circuits, particularly given the proposed wide channel band-widths of handset transceivers compared to, for example, DAMPS, and PDC systems. In order to realise a power efficient transceiver design, some form of linearisation is therefore required. The linearisation arrangement itself should be low power, capable of broad band linearisation (up to 5 MHz for UMTS/UTRA), frequency flexible, preferably multiband, and capable of achieving and maintaining high levels of linearity improvement when used to reduce distortion caused by highly non-linear power amplifiers (eg class C amplifiers), such as may be used in transceiver circuits.
The trend in base station technology is towards adoption of xe2x80x9csoftware radioxe2x80x9d techniques, that is, base station architectures in which all of the modulation parameters, ramping, framing, etc. take place for all channels at base band in the digital domain. The combination of all channels, at appropriate frequency offsets from one another, can also be performed at base band and the whole channel spectrum up-converted to the transmission frequency in a single block for multi-carrier power amplification and transmission from a single antenna.
According to one aspect, the invention provides an arrangement for reducing distortion of an output signal provided by signal handling means in response to an input signal, the arrangement comprising feedforward means which derives a feedforward signal from the input signal and combines it with the output signal to reduce distortion thereof, and predistorting means which predistorts the input signal prior to the signal handling means to counter distortion caused by the signal handling means, wherein the predistorting means also derives a reference signal from the input signal for use in controlling the feedforward signal. An arrangement of this type can handle digital base band signals (such as may be provided by a software radio) and perform the necessary frequency conversion and amplification, with reduced distortion, to provide an output signal for transmission from an antenna. Further, this arrangement allows the benefits of input signal predistortion and feedforward correction of the output signal to be realised independently, thus combining the distortion reduction effects of each without any degradation of either.
Advantageously, the reference signal may contain components which correspond to some or all of the intermodulation distortion components which can be produced by the signal handling means. Preferably, the reference signal is generated in the digital domain. The reference signal may be created to an arbitrarily high degree of accuracy, which leads to improved control of the feedforward signal.
In a preferred embodiment, the arrangement additionally comprises control means for adapting the feedforward signal using feedback derived from the output signal. Preferably, this feedback is extracted from the output signal after its combination with the feedforward signal. The feedback may be correlated with the reference signal provided by the predistorting means to produce a control signal for the feedforward means.
In a preferred embodiment, the arrangement also comprises means for adapting the predistortion applied by the predistorting means using feedback derived from the output signal.
Advantageously, the feedforward signal is produced by performing a subtraction on the input signal and a feedback signal derived from the output signal. Preferably, this feedback signal is taken from the output signal before its combination with the feedforward signal. The subtraction process preferably uses the input signal prior to its predistortion. The subtraction may be performed in the digital domain, or, alternatively, in the analogue domain.
According to another aspect, the invention provides a method for reducing distortion of an output signal provided by signal handling means in response to an input signal, the method comprising a feed forward step of deriving a feed forward signal from the input signal and combining it with the output signal to reduce distortion thereof, a predistorting step comprising predistorting, using predistorting means, the input signal prior to the signal handling means to counter distortion caused by the signal handling means, and a generating step of generating a reference signal from the input signal using the predistorting means for use in controlling the feed forward signal.